1. Technical Field
The present invention relates to a webbing take-up device that takes up a webbing for restraining a vehicle occupant to a take-up shaft and more particularly, to a webbing take-up device that rotates a take-up shaft by the driving force of a motor.
2. Related Art
In conventional webbing take-up devices, a speed reduction mechanism is interposed between a take-up shaft to take up a webbing and a motor (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 2006-282097). A clutch wheel is provided in the speed reduction mechanism. The clutch wheel includes a base member that can be connected to the take-up shaft and a gear member that receives the rotation force of the motor and rotates. A torque limiter (an overload release mechanism) is provided between the base member and the gear member. The overload release mechanism includes internal teeth that are formed in an inner circumferential portion of the gear member, a plate spring member that is mounted to the base member, and a roller that is urged in an engagement direction with the internal teeth by the urging force of the plate spring member.
In the overload release mechanism that has the above configuration, if the rotation force of a predetermined value or more acts between the gear member and the base member, the roller crosses over the internal teeth of the gear member while elastically deforming the plate spring member, and relative rotation of the gear member and the base member is enabled. Thereby, when the excessive rotation force acts on the take-up shaft via the webbing, a power transmission path of the motor and the take-up shaft is cutout.
Meanwhile, in the webbing take-up device that has the above configuration, the gear member and the base member are disposed on the same axis as the take-up shaft, and the excessive rotation force that acts on the take-up shaft is directly input to the overload release mechanism. The roller and the plate spring member, that constitute the overload release mechanism, are held in the base member. When the overload release mechanism is operated, the internal teeth of the gear member and the roller slide in contact. In order to secure durability of the gear member, the gear member may need to be formed of a metal material. For this reason, a reduction gear and the like, that transmits the rotation force of the motor to the gear member, may need to be formed of a metal material, which results in increasing a manufacturing cost and weight of the device.
In the overload release mechanism that has the above configuration, a tip end of the plate spring member that is mounted to one of a motor-side rotation body and a take-up shaft-side rotation body may be directly engaged with ratchet teeth (internal teeth or external teeth) provided in the other. In this overload release mechanism, at the time of relative rotation of one of the motor-side rotation body and the take-up shaft-side rotation body with respect to the other (at the time of operating of the overload release mechanism), since an angular portion of the tip end of the plate spring member is slid with the ratchet teeth, it may be problem that durability of the ratchet teeth is deteriorated. Accordingly, the tip end side of the plate spring member is curved in an approximately C shape in side view and an outer circumferential surface (curved surface) of the curved portion is engaged with the ratchet teeth, thereby securing the durability of the ratchet teeth.
However, when the tip end side of the plate spring member is curved and the curved portion is formed, there is a limitation in decreasing radius of curvature thereof. For this reason, in order to sufficiently secure engagement allowance (maximum bending amount of the plate spring member) of the tip end side (curved portion) of the plate spring member and the ratchet teeth and apply a predetermined torque (so-called overload) to the relative rotation of one of the motor-side rotation body and the take-up shaft-side rotation body with respect to the other, the height of the ratchet teeth needs to be increased. For this reason, an outer diameter of the other (motor-side rotation body or the take-up shaft-side rotation body) that is provided with the ratchet teeth increases. As a result, the size of the overload release mechanism may radially increase.